Titanium material

ABSTRACT

An object of the present invention is to provide a titanium plate having high strength and excellent workability. In order to achieve this object, the present invention provides a titanium material having an iron content of 0.60% by mass or less and an oxygen content of 0.15% by mass or less, with the balance being titanium and unavoidable impurities, the titanium material having a worked structure formed by working accompanied by plastic deformation and a recrystallized structure formed by annealing after the working, wherein the titanium material is formed such that the average particle size of crystal grains of the recrystallized structure is 1 μm or more and 5 μm or less, and the area of a non-recrystallized part in the cross-sectional area of the titanium material is more than 0% and 30% or less.

TECHNICAL FIELD

The present invention relates to a titanium material, more particularly to a titanium material excellent in strength and workability.

BACKGROUND ART

Conventionally, plate-shaped and bar-shaped members formed from materials such as titanium alloys and pure titanium have been widely used.

For example, a plate-shaped titanium material (hereinafter also referred to as a “titanium plate”) has been widely used for industrial products, wherein the titanium plate is subjected to various workings accompanied by plastic deformation such as folding, bulging, and drawing to form various products.

The titanium plate which is subjected to such working is demanded to have excellent workability.

Further, recently, the reduction in the thickness of a titanium plate has been demanded in terms of reducing material cost, reducing the weight of a product, and the like.

As a result, improvement in the strength of a titanium plate has increasingly been demanded.

However, conventionally, the workability and the strength of a titanium plate are in a trade-off relation, and it is difficult to simultaneously satisfy these properties.

That is, conventional titanium plates have a problem that fabrication becomes difficult (poor in workability) with the increase in yield strength.

With respect to the above subject, the following Patent Literature 1 shows the results of the evaluation of workability of titanium thin plates having different components and crystal grain sizes in a cupping test and describes that the finer the crystal grain, the better the workability is (page 103, from line 5).

Further, the following Patent Literature 1 discloses a method for producing a pure titanium thin plate and describes the production of a pure titanium thin plate having a reduced gloss surface, including performing final annealing in the atmospheric air by the continuous annealing at (600 to 800)° C.×(2 to 5) minutes, then performing pickling treatment, and adjusting the average crystal grain size (hereinafter referred to as particle size) of the product to 3 to 60 μm.

Further, the following Patent Literature 2 discloses pure titanium for building materials, a pure titanium plate, and a method for producing the same and describes a titanium material for building materials which contains 900 ppm or less of oxygen and 100 ppm or more and 600 ppm or less of Fe, wherein the content of Ni and Cr is restrained.

Furthermore, Patent Literature 2 describes a titanium material for building materials having an average crystal grain size of 70 μm or less which has been subjected to pickling treatment with an aqueous nitric hydrofluoric acid solution after cold rolling and annealing.

However, these Patent Literatures 1 and 2 show almost no data in which a titanium material having a fine crystal grain size of 5 μm or less has been evaluated, and Patent Literature 2 shows an Example in which the crystal grain size is 3 μm but at the same time describes in the paragraph [0026] that “In actual production, the lower limit will be about 5 μm”, which is a negative description on a crystal grain size of 5 μm or less.

This is probably because these literatures aim at obtaining an excellent titanium material for building materials having a reduced gloss, and the workability in bulging, deep drawing, and the like has not been sufficiently investigated.

Further, the following Patent Literature 4 discloses a titanium plate excellent in workability, which has a low strength (yield strength) irrespective of having excellent workability and cannot satisfy both workability and strength at the same time.

CITATION LIST Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 63-103056 Patent Literature 2: Japanese Patent Laid-Open No. 9-3573 Patent Literature 3: Japanese Patent Laid-Open No. 2006-316323 Patent Literature 4: Japanese Patent Laid-Open No. 63-60247 Non Patent Literature

Non Patent Literature 1: “Titanium”, Vol. 57, No. 2 (issued by the Japan Titanium Society, in April 2009)

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a titanium plate having high strength and excellent workability.

Solution to Problem

Although the strength (yield strength) of a titanium material can be increased by mainly adding oxygen (O) and iron (Fe), but when these are added, ductility will be reduced to thereby reduce workability.

For example, since a titanium material specified in JIS class 1 has a low content of oxygen and iron, a titanium plate using the material of JIS class 1 generally has a low strength (yield strength) but is excellent in ductility and excellent in workability.

When a titanium material of JIS class 2 having a higher content of oxygen and iron than the titanium material of JIS class 1 is used, the resulting titanium material will have a higher strength (yield strength) than the titanium material in which the titanium material of JIS class 1 is used, while it will tend to have a reduced ductility to reduce workability.

Titanium materials of JIS class 3 and class 4 having a much higher content of oxygen and iron have much higher strength (yield strength) but have a much reduced ductility to greatly reduce workability.

That is, strength (yield strength) and workability have a certain relationship (hereinafter, this relationship is also referred to as “strength (yield strength)-workability” balance).

Incidentally, plate materials and wire materials prepared by using titanium materials are formed by subjecting the materials to working accompanied by plastic deformation such as rolling and wire drawing.

These plate materials and wire materials subjected to working accompanied by plastic deformation generally have the inner part in which a worked structure is formed in the state as it is, and therefore, they are subjected to a step called final annealing in order to recrystallize the structure before they are supplied to the market.

For example, a titanium plate is subjected to working such as cold rolling to adjust the thickness to a predetermined value and then subjected to batch annealing, continuous annealing, or the like to recrystallize the worked structure in the inner part to form equi-axed crystal grains (hereinafter, referred to as “recrystallized grains”).

These recrystallized grains greatly grow with the passage of annealing time and the like, and in particular, in the period immediately after the initiation of recrystallization where the particle size of the recrystallized grains is small, the growth rate of the recrystallized grains will be high and they will grow to a large particle size exceeding 5 μm in a relatively short time.

When the recrystallized grains grow to such a size, a non-recrystallized part (worked structure) will not remain, but only the equi-axed structure based on the recrystallized grains will generally be formed in the inner part of the titanium material.

As a result of extensive and intensive investigations to achieve the above-described object, the present inventors have found that improvement in the strength (yield strength) of a titanium material can be achieved by adjusting a structure (refining of crystal grains by leaving a non-recrystallized part) to which attention has not been paid as a means for improving strength (yield strength).

Specifically, the present inventors have completed the present invention by subjecting a commercially pure titanium plate which has been cold-rolled to a predetermined thickness to final annealing in a vacuum using an electric furnace; making various titanium plates having different structures on an experimental basis by changing the temperature and time thereof; and evaluating the strength (yield strength) and workability (ductility) thereof by a tensile test and an Erichsen test.

As a result of the evaluation, it has been found that although strength (yield strength) tends to increase and workability (Erichsen value) tends to be reduced with the decrease in the size of crystal grains, the Erichsen value is not significantly reduced provided that the average particle size of the recrystallized grains is a predetermined size or less, and the “strength (yield strength)-workability balance” can be improved compared with conventional titanium materials.

Further, there has been a case where even if the average crystal grain size of the recrystallized grains is a predetermined size or less, workability (Erichsen value) is reduced, and therefore, the “strength (yield strength)-workability balance” cannot be improved compared with conventional titanium materials.

As a result of the investigation of the microstructure of this titanium plate in detail, many non-recrystallized parts have been observed in addition to the grains recrystallized by final annealing.

The “strength (yield strength)-workability balance” has been investigated based on the amount of the non-recrystallized part, and it has been found that workability is extremely reduced if the area rate of the non-recrystallized part in the cross-sectional area of the titanium plate exceeds 30%.

Note that, herein, the non-recrystallized part means a part in which a worked structure subjected to plastic working remains.

Specifically, the present invention related to a titanium material for achieving the above object is characterized in that the titanium material has an iron content of 0.60% by mass or less and an oxygen content of 0.15% by mass or less, with the balance being titanium and unavoidable impurities, the titanium material having a worked structure formed by working accompanied by plastic deformation and a recrystallized structure formed by annealing after the working, wherein the titanium material is formed such that the average particle size of crystal grains of the recrystallized structure is 1 μm or more and 5 μm or less, and the area of a non-recrystallized part in the cross-sectional area of the titanium material is more than 0% and 30% or less.

Advantageous Effect of Invention

The present invention can provide a titanium material having high strength and excellent workability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photomicrograph showing the microstructure of the titanium plate of Example observed with a transmission electron microscope (a non-recrystallized part is observed in a part between recrystallized grains).

FIG. 2 is a graph showing the relationship between the yield strength and the Erichsen value.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a preferred embodiment of the titanium material according to the present invention will be described taking a titanium plate as an example.

The titanium plate in the present embodiment is formed from a titanium material having an iron (Fe) content of 0.60% by mass or less and an oxygen (O) content of 0.15% by mass or less, with the balance being titanium (Ti) and unavoidable impurities.

This titanium plate is formed by working accompanied by plastic deformation followed by annealing and has, in the inner part thereof, a worked structure accompanying the working and a recrystallized structure accompanying the annealing, wherein the titanium plate is formed such that the average particle size of crystal grains of the recrystallized structure is 1 μm or more and 5 μm or less, and the area of a non-recrystallized part in the cross-sectional area of the titanium plate is more than 0% and 30% or less.

As described above, the iron (Fe) is contained at a percentage of 0.60% by mass or less.

Note that the upper limit of Fe is 0.60% by mass because Fe is a β-phase-stabilizing element in a titanium material, and if the content of Fe exceeds 0.60% by mass, many β-phases may be produced in the structure constituting the titanium plate in addition to the α-phase.

That is, since ductility is greatly reduced or corrosion resistance is reduced depending on the size of the β-phase formed, it is important to keep the content of Fe contained in the titanium material which forms the titanium plate of the present embodiment at 0.60% by mass or less in terms of forming a titanium plate having high strength and excellent workability.

Note that although the lower limit of the Fe content is not necessarily demanded in terms of forming a titanium plate having high strength and excellent workability, an expensive and high purity titanium sponge must be used as a raw material if a titanium plate having a Fe content of less than 0.01% by mass is intended to be used, which may increase the material cost of the titanium plate.

Therefore, the content of Fe is preferably 0.01% by mass or more and 0.60% by mass or less in terms of the cost of the titanium plate and the like.

For example, in the Kroll process, a titanium material having an Fe content of 0.60% by mass or more is generally formed only in a small region near the vessel.

Therefore, most of the titanium sponge obtained by the Kroll process can be used because the titanium plate in the present embodiment has a content of iron as a component in the range of 0.01 to 0.60% by mass.

That is, the titanium plate of the present embodiment can be said to be suitable as a consumption material in that almost no restriction is added to the use part of the titanium sponge.

The oxygen (O) is contained in the titanium material in a content of 0.15% by mass or less.

The O content of the titanium material forming the titanium plate of the present embodiment is 0.15% by mass or less because if the O content exceeds 0.15% by mass, the strength of the titanium plate may be excessively improved to prevent the workability thereof from being sufficiently imparted thereto even if improvement in the “strength-workability balance” is intended to be achieved by reducing the size of crystal grains, thus making it difficult to form a titanium plate suitable for working such as bulging and deep drawing.

Note that although the lower limit of the O content is not particularly provided, a titanium plate may have to be produced using an expensive and high purity titanium sponge as a raw material if the O content of the titanium material constituting the titanium plate is intended to be set to less than 0.015% by mass.

Therefore, the O content is preferably 0.015% by mass or more and 0.15% by mass or less.

Further, it is important that unavoidable impurities such as carbon (C), nitrogen (N), and hydrogen (H) are each contained in a content corresponding to JIS class 2 or less for the purpose of ensuring good workability in fabrication.

More specifically, it is important that the content of C, N, and H is each less than 0.02% by mass.

Further, the content of C is preferably 0.01% by mass or less, the content of N is preferably 0.01% by mass or less, and the content of H is preferably 0.01% by mass or less.

Although a lower limit is not provided in the above content of C, N, and H from the point of view of the workability of a titanium plate, the production cost of the titanium plate may be significantly increased if the content is intended to be extremely reduced.

From the point of view of preventing such cost increase, the C content is preferably 0.0005% by mass or more, the content of N is preferably 0.0005% by mass or more, and the content of H is preferably 0.0005% by mass or more.

As described above, the titanium plate of the present invention has a worked structure and a recrystallized structure in the inner part thereof and is formed such that the average particle size of crystal grains of the recrystallized structure is 1 μm or more and 5 μm or less, and the area of a non-recrystallized part in the cross-sectional area of the titanium plate is more than 0% and 30% or less.

The upper limit of the average particle size of the recrystallized structure is 5 μm because if the average crystal grain size of equi-axed cc-grains produced by recrystallization exceeds 5 μm, the effect of the refining of crystal grains will be small, making it difficult to achieve excellent “strength-workability balance”.

Further, the lower limit is 1 μm because if the titanium plate is subjected to working (rolling, forging, or the like) in actual production (by an industrially feasible method) followed by annealing to obtain an average crystal grain size of less than 1 μm, the area rate of the non-recrystallized part (worked structure) to be described below will increase, which extremely increases strength but greatly reduces ductility, making it difficult to achieve excellent “strength-workability balance”.

The non-recrystallized part is formed from a worked structure in which a titanium plate is plastically deformed by working (cold rolling, forging, or the like) to collapse crystal grains, and the strength of the titanium plate can be improved by allowing the worked structure to remain in the titanium plate.

A titanium plate comprising a worked structure formed by cold rolling or the like has high strength while its ductility is very small.

Therefore, the worked structure has conventionally been recrystallized by annealing to form an equi-axed structure, and sufficient annealing time has been provided to such an extent that the worked structure does not remain in the titanium plate.

On the other hand, with respect to the titanium plate in the present embodiment, the worked structure is allowed to remain in the titanium plate by employing annealing conditions to be described below, and, moreover, the particle size of recrystallized grains is adjusted as described above.

It is important in terms of obtaining excellent “strength-workability balance” that the non-recrystallized part (worked structure) is provided so that the ratio of the area thereof to the cross sectional area of the titanium plate is 30% or less.

If the area rate of the non-recrystallized part is higher than 30%, the strength of the titanium plate will be higher, but the ductility will be reduced, making it difficult to allow the titanium plate to exhibit excellent workability.

As a result, it may be impossible to obtain excellent “strength-workability balance.”

The area rate of the non-recrystallized part is preferably 10% or less in terms of more reliably imparting the excellent “strength-workability balance” to the titanium plate.

Note that although the lower limit is not particularly limited, the particle size of recrystallized grains will rapidly increase if the non-recrystallized part is lost (the area rate is 0%).

Therefore, the area rate of the non-recrystallized part is preferably 0.1% or more in that the particle size of recrystallized grains can be more reliably adjusted within the range as described above.

The method for adjusting the particle size of the recrystallized grains and forming the non-recrystallized part as described above includes a method in which the titanium plate is adjusted to a desired thickness in a common rolling process and the like and then subjected to final annealing in a predetermined condition.

The annealing technique which can be employed in the final annealing can be roughly classified into a continuous type and a batch type.

Among these, a continuous type final annealing is a method of annealing by spreading a cold-rolled coil and passing a titanium plate at a constant speed through an annealing furnace, and the method can control the holding time of heating temperature by the plate-passing speed.

In the final annealing of conventional titanium plates, in the case of the continuous type, the heating temperature is 700 to 800° C., and the heating time is from several tens of seconds to about 2 minutes.

On the other hand, a batch type final annealing is a method of heating the coil of a titanium plate in an annealing furnace in the state of a coil as it is, wherein the titanium plate is slowly heated, in order to reduce the difference in the application of heat between the outer part and the inner part of the coil, and its cooling rate is also very slow.

In the final annealing of conventional titanium plates, in the case of the batch type, the heating temperature is 550 to 650° C., and the heating time is from about 3 hours to 30 hours.

On the other hand, the final annealing performed when producing the titanium plate of the present embodiment is preferably performed, for example, in a continuous system, under heating conditions at a temperature of 580° C. or more and less than 600° C. for 1 minute or more and 10 minutes and less, or under heating conditions at a temperature of 600° C. or more and 650° C. or less for 10 seconds or more and 2 minutes or less.

The time period of 10 seconds or more is selected as a preferred heating condition because if the time for holding the temperature is shorter than 10 seconds, a proper range of operation conditions such as plate-passing speed and heating temperature to perform predetermined annealing to a titanium plate will be very narrow, which requires highly accurate control of an apparatus or its operation.

On the other hand, a condition of 10 minutes or less is preferred as the heating time because if the holding time exceeds 10 minutes, the plate-passing speed must be reduced, thus reducing the productivity.

Further, a temperature of 580° C. or more is selected as a preferred condition of heating temperature because if the heating temperature is lower than 580° C., it will be difficult to cause a predetermined recrystallization in a titanium plate in a holding time of 10 minutes or less, and the area rate of the non-recrystallized part will exceed 30% in many cases.

Furthermore, the heating temperature of 650° C. or less is selected because if the temperature is higher than 650° C., the recrystallization of a titanium plate may have been completed even in a heating time of 10 seconds, and recrystallized grains may grow to an average particle size of 5 μm or more.

Further, the final annealing performed when producing the titanium plate of the present embodiment is preferably performed under heating conditions at a temperature of 420° C. or more and less than 550° C. for 3 hours or more and 50 hours or less, when it is a batch type.

A condition of 3 hours or more is preferred as the heating time because if the heating time is shorter than 3 hours, the temperature in the inner part of a coil may not reach a predetermined temperature depending on the size of the coil.

On the other hand, a condition of 50 hours or less is preferred as the heating time because if the heating time exceeds 50 hours, the time required for annealing will be excessively long, thus reducing the productivity of the titanium plate.

Further, a heating temperature of 420° C. or more is preferred because if the heating temperature is lower than 420° C., it will be difficult to cause a predetermined recrystallization in a titanium plate in a holding time of 50 hours or less, and the area rate of the non-recrystallized part will exceed 30% in many cases.

Or it is because several annealing furnaces (heating equipment) must be possessed in order to ensure a predetermined production volume, which increases the cost of equipment and requires a large space for installing the annealing furnaces.

Note that, in a batch type, since the titanium plate is heated in the state of a coil, the temperature increasing rate is different between the outer part and the inner part of the coil, and the time until the temperature reaches a target temperature is also different.

Depending on the size of the coil, the heating temperature, and the heating capacity of the annealing furnace, the time until the temperature reaches a target temperature generally differs by tens of minutes to several hours.

Therefore, it is important to heat the coil to a temperature range where the size of recrystallized grains does not greatly differ even if the heating time differs to some extent, that is, important to a temperature range where the growth rate of recrystallized grains is slow.

Further, the heating temperature is preferably less than 550° C. because since the growth rate of recrystallized crystal grains is high at a temperature of 550° C. or more, when the heating time is shortened in accordance with the outer part of the coil, a target temperature may not be reached in the inner part of the coil, leading to a state where a non-recrystallized part which is not recrystallized may be in an amount exceeding 30%; conversely, when the heating time is lengthened in accordance with the inner part of the coil, the recrystallized grains may excessively grow in the outer part of the coil, leading to an average crystal grain size of 5 μm or more.

Note that the final annealing of either a continuous type or a batch type is desirably performed in a vacuum or in an inert gas atmosphere.

A titanium plate having excellent “strength-workability balance” can be obtained by adjusting the average particle size of recrystallization and the residual percentage of the non-recrystallized part (worked structure) with the annealing conditions as described above.

Note that although not described in detail here, a known matter in a conventional titanium plate and titanium plate production method can also be employed in the present invention in the range which does not significantly impair the effect of the present invention.

Further, although a titanium plate is mentioned as an example of a titanium material in the present embodiment, a titanium material of various forms such as a wire material, a bar material, and a tubing material is the same as the titanium plate in that excellent “strength-workability balance” is exhibited, and these titanium materials also fall within the scope intended by the present invention.

EXAMPLES

Next, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these.

<Evaluation 1> (Sample Nos. 1 to 45) (Preparation of Test Pieces)

An ingot (140 mm in diameter) was prepared by small-sized vacuum arc melting, and the ingot was heated to 1050° C. and then forged to prepare a slab having a thickness of 50 mm.

The slab was hot-rolled at 850° C. to a thickness of 5 mm and then annealed at 750° C., and the scale on the surface of the annealed slab was removed by shot peening and pickling to prepare a plate material.

The plate material was further cold-rolled to prepare a plate-shaped sample (titanium plate) having a thickness of 0.5 mm.

The titanium plate having a thickness of 0.5 mm was subjected to final annealing at a temperature of 400 to 800° C. for 48 hours or less in an argon gas atmosphere to prepare a test piece in which crystal grains have been adjusted.

(Measurement of Components)

The amounts of iron and oxygen contained in the titanium plate were measured using the plate material after hot-rolling from which the surface scale was cut.

The iron content was measured according to JIS H1614, and the oxygen content was measured according to JIS H1620.

(Measurement of Tensile Strength)

Further, the tensile strength of the test piece (titanium plate) in which the crystal grain size has been adjusted as described above was measured according to JIS Z 2241.

(Evaluation of Workability)

Furthermore, the workability of the test piece (titanium plate) in which the crystal grain size has been adjusted as described above was evaluated.

The evaluation was performed by the measurement of the Erichsen value using graphite grease as a lubricant according to JIS Z2247.

(Investigation of Structure)

The microstructure of the titanium plate was observed to obtain structural photographs of crystal grains (recrystallized α-grains) and a non-recrystallized part (worked structure).

Note that an optical microscope or a transmission electron microscope was used for the observation.

An example of the structural photograph observed with a transmission electron microscope is shown in FIG. 1 (microstructure of sample No. 28).

In this structural photograph, recrystallized α-grains and a non-recrystallized part are observed.

(In the photograph shown in FIG. 1, a place as indicated by “A” is the non-recrystallized part.)

This photograph was determined for the area other than the non-recrystallized part using image analysis software to determine the average area of recrystallized α-grains, and the diameter of a circle having the same area as the average area was determined by calculation to define the average particle size of recrystallized grains.

Further, the area rate of the non-recrystallized part was determined from the area of the non-recrystallized part.

The results of the above are shown in Table 1.

TABLE 1 Average crystal grain O Fe size of Area rate of Yield Erichsen Sample content content Annealing condition recrystallized non-recrystallized strength value No. (mass %) (mass %) Temp. Time grains (μm) part (%) (MPa) (mm) 1 0.021 0.017 450 8 hr 2.3 25 190 13.9 2 0.021 0.017 600 1 min 3.9 2 162 14.5 3 0.024 0.253 600 110 sec 2.2 16 352 10.9 4 0.024 0.253 630 110 sec 2.8 11 305 11.7 5 0.024 0.253 650 1 min 3.4 5 268 12.1 6 0.030 0.022 450 8 hr 2.0 23 240 12.9 7 0.030 0.022 450 48 hr 2.6 2 229 13.1 8 0.030 0.022 480 8 hr 2.3 10 236 13.0 9 0.030 0.022 480 24 hr 2.8 5 230 13.2 10 0.030 0.022 480 32 hr 2.9 3 224 13.3 11 0.030 0.022 480 48 hr 3.1 1 225 13.3 12 0.030 0.022 500 8 hr 3.3 3 217 13.4 13 0.030 0.022 520 4 hr 4.5 0.5 210 13.6 14 0.030 0.022 600 1 min 3.5 2 205 13.4 15 0.035 0.027 600 10 sec 3.6 15 261 12.3 16 0.035 0.027 600 30 sec 4.1 3 255 12.5 17 0.035 0.027 600 1 min 4.2 1 262 12.4 18 0.035 0.027 630 10 sec 4.4 2 264 12.4 19 0.035 0.027 630 30 sec 4.7 1 250 13.3 20 0.053 0.217 650 1 min 4.2 3 310 11.5 21 0.066 0.377 650 1 min 3.4 4 336 11.0 22 0.066 0.377 650 10 sec 4.9 1 301 11.7 23 0.068 0.059 450 8 hr 1.8 21 428 9.4 24 0.068 0.059 500 8 hr 3.2 2 356 10.7 25 0.068 0.059 600 1 min 3.3 2 345 10.8 26 0.068 0.059 650 10 sec 4.9 0.2 313 12.1 27 0.042 0.024 425 24 hr 1.8 26 360 10.4 28 0.042 0.024 450 24 hr 2.6 13 304 12.0 29 0.042 0.024 480 24 hr 3.3 4 264 12.0 30 0.042 0.024 500 24 hr 4.6 1.5 240 12.5 31 0.021 0.017 600 4 hr 26 0 107 14.0 32 0.030 0.022 600 1 hr 12 0 172 12.7 33 0.030 0.022 600 4 hr 23 0 159 13.0 34 0.030 0.022 750 1 min 46 0 148 13.2 35 0.035 0.027 800 1 min 82 0 146 13.4 36 0.053 0.217 800 5 min 17.2 0 430 8.0 37 0.066 0.377 800 15 hr 21 0 266 9.2 38 0.068 0.059 750 1 min 42 0 199 11.7 39 0.068 0.059 800 1 min 50 0 189 12.2 40 0.068 0.059 800 15 min 75 0 192 11.5 41 0.160 0.065 750 10 min 28 0 346 8.2 42 0.209 0.104 750 10 min 22 0 411 7.6 43 0.030 0.022 450 1 hr 1.8 43 263 8.6 44 0.066 0.377 500 1 hr 2.3 35 238 10.4 45 0.042 0.024 400 24 hr 1.5 45 414 6.9

The above Sample Nos. 1 to 30 each have an average particle size of recrystallized grains of 5 μm or less, and in each of these samples, a non-recrystallized part is observed at an area rate of less than 30% in the cross section of the titanium plate; and Sample Nos. 31 to 42 are in the state where the non-recrystallized part does not remain, like conventional titanium plates.

Further, Sample Nos. 43 to 45 have been obtained by adjusting annealing conditions so that the non-recrystallized part is intentionally allowed to remain, wherein the non-recrystallized part has been allowed to remain in the state where the area rate exceeds 30%.

The above Sample Nos. 1 to 30 and Nos. 31 to 42 have been obtained by adjusting the size of crystal grains (circle-equivalent average grain size of the α-phase) and the amount of the non-recrystallized part with the difference between annealing conditions, irrespective of using titanium materials in which oxygen content and iron content are almost the same.

As shown in Table 1, the average particle size can be suppressed small and high yield strength is exhibited by containing the non-recrystallized part.

In the above evaluation, workability (Erichsen value) generally tends to decrease as the yield strength increases, but when the samples having comparable workability (Erichsen value) are compared with each other, it is found that the yield strength of these samples is increased and these samples have high strength by allowing the non-recrystallized part to be present (for example, refer to the comparison of Sample No. 1 with No. 31, No. 9 with No. 34, and No. 15 with No. 39).

That is, it is found that when crystal grains have a size of 5 μm or less and a non-recrystallized part is present in an amount of 30% or less, the “yield strength-workability balance” is good.

On the other hand, when the area of the non-recrystallized part is more than 30% after the final annealing, workability (Erichsen value) is greatly reduced, as shown in Sample Nos. 43 to 45.

These results have also shown that the present invention can provide a titanium plate having high strength and excellent workability.

<Evaluation 2>

(Sample Nos. A to H)

(Actual Machine Test) (Preparation of Test Coil)

An ingot (750 mm in diameter) was prepared by vacuum arc melting, and the ingot was heated to 850 to 1000° C. and then forged to prepare a slab having a thickness of 170 mm.

The slab was heated to a temperature of 850° C. and then hot-rolled to a thickness of 3.5 mm, and the hot-rolled plate was annealed at a temperature of 750° C., followed by removing the scale on the surface of the annealed slab by shot peening and pickling to prepare a hot-rolled coil.

The hot-rolled coil was cold-rolled to obtain a cold-rolled coil having a thickness of 0.4 to 0.8 mm.

Oil and fat such as cold rolling oil were removed from the cold-rolled coil by cleaning, and the resulting cold-rolled coil was inserted in a vacuum annealing furnace.

The inside of the vacuum annealing furnace in which the cold-rolled coil was accommodated was evacuated and then replaced with argon gas, and in the furnace, the cold-rolled coil was subjected to a batch type annealing in which it was heated to 450 to 650° C. and held for 4 to 36 hours to adjust the size of recrystallized grains

In order to evaluate “Measurement of Components”, “Measurement of Tensile Strength”, “Evaluation of Workability”, and “Investigation of Structure” in the same manner as in the above Evaluation 1, samples of required size were taken from the obtained titanium plate and subjected to the evaluations as described above. The results are shown in Table 2.

TABLE 2 Average crystal grain O Fe Annealing size of Area rate of Yield Erichsen Sample content content condition recrystallized non-recrystallized strength value No. (mass %) (mass %) Temp. Time grains (μm) part (%) (MPa) (mm) A 0.028 0.019 500 24 hr 3.6 1 213 13.5 B 0.032 0.024 480 24 hr 2.6 5 232 13.1 C 0.035 0.022 480 24 hr 2.4 4 248 12.8 D 0.058 00234 450 36 hr 2.3 3 385 10.1 E 0.068 0.033 450 36 hr 2.4 4 401 9.9 F 0.022 0.014 600  4 hr 25 0 110 14.0 G 0.030 0.018 630 24 hr 45 0 149 13.2 H 0.041 0.028 650  4 hr 55 0 166 12.8

The above Sample Nos. A to E each have an average particle size of recrystallized grains of 5 μm or less, and in each of these samples, a non-recrystallized part is observed at an area rate of less than 30% in the cross section of the titanium plate; and Sample Nos. F to H are in the state where the non-recrystallized part does not remain, like conventional titanium plates.

In the above Sample Nos. A, B, and C, there have been obtained titanium plates having a yield strength of 200 MPaa or more and having excellent workability in which the Erichsen value is about 13 mm.

Further, in Sample Nos. D and E, there have been obtained titanium plates not only having high strength in which the yield strength is about 400 MPa but also having good workability in which the Erichsen value is about 10 mm

On the other hand, Sample Nos. F to H are excellent in workability but have insufficient strength in which the yield strength is less than 200 MPa.

These results have also shown that the present invention can provide a titanium plate having high strength and excellent workability. 

1. A titanium material having an iron content of 0.60% by mass or less and an oxygen content of 0.15% by mass or less, with the balance being titanium and unavoidable impurities, the titanium material having a worked structure formed by working accompanied by plastic deformation and a recrystallized structure formed by annealing after the working, wherein the titanium material is formed such that the average particle size of crystal grains of the recrystallized structure is 1 μm or more and 5 μm or less, and the area of a non-recrystallized part in the cross-sectional area of the titanium material is more than 0% and 30% or less. 